1. Field of the Invention
The present invention relates to a break-in detection sensor for detecting intrusion into a building or premises by an optical fiber sensor and, more particularly to, a system in which an optical fiber detection sensor of an FBG type is laid down on a side of a fence or the top thereof to detect intrusion.
2. Description of the Related Art
In recent years, security against terrorism or illegal intrusion in airports, harbors, defense facilities, and other important places attracts attention. Various break-in detection apparatuses or break-in detection systems for detecting intrusion into buildings or premises are proposed and executed.
As detection sensors used in the apparatuses and systems of this type include a vibration sensor, an infra-red ray interception sensor, an electric field interception sensor, a mechanical tension sensor, and an abnormal state sensor for surveillance image obtained by a surveillance monitor are known. Furthermore, an optical fiber sensor using an optical fiber is proposed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2001-296111).
As surveillance systems operated in coordination with the detection sensors, a recording method of ITV camera images and remote monitoring, an image analysis method, a method using alarm generation by an alarm unit and wireless communication are known.
As other detection sensors using optical fibers, optical fiber detection sensors of an FBG (Fiber Brag Grating) type and an OTDR (Optical Time Domain Reflectometry) type are known.
FIG. 3 shows the principle of the FBG type fibro-optic detection sensor. As shown in FIG. 3, grating sections (FBG) 101 having different fiber glass refraction indexes are provided at predetermined intervals in longitudinal cross section through an optical fiber 100. The grating sections 101 resonate and reflect only components having the wavelength of two times the interval λL out of pulse lights coming from an optical signal generator 102. The thus reflected light has a wavelength shifted in proportion to stretch strain in the grating sections 101. The reflected light component is guided by a half mirror 103 through a narrow band variable filter 104 to a light receiver 105 for detection. By checking the degree of wavelength shifts (frequency shifts), it can be detected whether or not the stretch strain in the optical fiber exceeds a predetermined value. When this detection is executed, the positions of the grating sections 101 can be discriminated as positions of intrusion.
FIG. 4 explains the principle of an OTDR fiber-optic detection sensor. An optical fiber includes sections having different refraction factors. When light passes through the sections, the light is refracted and scattered due to the different refraction factors such that light rays having wave lengths equal to that of the incident light are reflected on an end of the optical fiber on which the light is incident. The OTDR fiber-optic detection sensor makes use of this Rayleigh scattering light, where a light pulse issued from a light pulse generator 106 is introduced into the optical fiber 107 before Rayleigh scattering light produced therein is guided out thereof via a half mirror 108 to be received by a receiver 109 where any optical fiber strain, displacement and disconnection points can be detected on the basis of the amount of light or the time required for reflection.
In the break-in detection apparatus using the FBG type fibro-optic detection sensor or the OTDR fibro-optic detection sensor, an optical fiber is laid down along a fence or a wall of premises or facilities to be detected to make it possible to detect intrusion. In particular, in the FBG type fibro-optic detection sensor, a plurality of FBGs having different reflection wavelengths are incorporated in a core section of one optical fiber to make it possible to simultaneously detect intrusion at a large number of positions.
This configuration has a structure in which FBG (Grating sections) 110A to 110N are incorporated in the core section of the optical fiber 110 at appropriate intervals and have different reflection wavelengths λ1, λ2, λ3, . . . . An optical signal generator 111 continuously or intermittently (pulsatively) generates an optical signal in a band including the reflection wavelengths held by the FBGs incorporated in the optical fiber 110. A half mirror 112 uses the optical signal from the optical signal generator 111 as an optical input to the optical fiber 110 to optically guide reflected lights from the FBGs of the optical fiber 110 to a narrow band variable filter 113. The narrow band variable filter 113 transmit the reflected waves from the FBGs at once to output these reflected waves to a photodetector 114. The photodetector 114 simultaneously converts the reflected waves into electric signals having equal frequencies or low frequencies obtained by multiplying the frequencies by 1/n. A wavelength shift detector 115 compares the frequency signals obtained by the photodetector 114 with a reference frequency signal from a reference frequency generator 116 to obtain pulse signals having timings sorted by reflection wavelengths the frequencies of which shift. An intrusion position determination section 117 determines a position of an FBG where a reflection wavelength shifts, i.e., an intrusion position on the basis of the timings of the pulse signals to obtain the output.